The present invention relates generally to magnetron sputtering devices, and particularly to an internally cooled target assembly for use in a magnetron sputtering device. More particularly, the present invention relates to an improved internally cooled target assembly configured to promote highly turbulent coolant flow through the target assembly to achieve efficient and uniform target cooling while minimizing the volume of coolant needed to cool the target assembly.
Magnetron sputtering devices have long been used by the semiconductor processing industry to coat substrates (e.g., silicon wafers) with various materials (e.g., aluminum, titanium, gold, etc.) during the manufacture of integrated circuits. Generally, in a sputtering device, the material to be deposited or sputtered on the substrate is contained in a target. The substrate is placed on a substrate support table in a sputtering chamber. Air in the sputtering chamber is evacuated and replaced with an inert gas such as argon, preferably at a low pressure. An electric field is established between an anode such as the walls that line the sputtering chamber and the target. The target acts as an electron source, or a cathode. Ions are formed when electrons collide with the inert gas. The ions are then drawn toward the target by the electric field. The ions impact the target with sufficient energy to dislodge, or sputter, atoms of target material into the sputtering chamber. The sputtered atoms travel from the target surface to the substrate, coating the substrate with a thin film of target material. The target erodes more quickly in regions where more ions impact the target. The cloud of free electrons, inert gas atoms, inert gas ions and sputtered atoms that exist near the target sputtering surface is termed a "plasma discharge."
The location of plasma discharge may be controlled by introducing a magnetic field adjacent to the sputtering surface of the target in the sputtering chamber. The magnetic field is generated by a rotating magnetic circuit located on the side of the target opposite the sputtering surface. The magnetic field acts to trap electrons in a desired region so that ionization is concentrated in that region. Creation of such a region adjacent to the target surface results in a corresponding region of plasma discharge and erosion in the target sputtering surface. The region of plasma discharge rotates with the magnetic circuit about an axis that is perpendicular to the target sputtering surface.
Uniformity of the thickness of the film of material deposited onto a substrate is an important parameter in magnetron sputtering. Film thickness uniformity has become increasingly important as the semiconductor processing industry reduces semiconductor device geometries to achieve greater device densities. At the same time that uniformity requirements have become more stringent, the use of larger, 200 mm semiconductor substrates has become increasingly common. The use of larger substrates makes it more difficult to meet tighter uniformity requirements.
Uniformity may be improved by increasing the diameter of the targets used in magnetron sputtering devices. Sputtering with a larger target provides a more even distribution of sputtered material in the vicinity of the substrate. It has been found that significant improvements in film thickness uniformity on 200 mm substrates may be achieved by using sputtering targets having diameters of about 350 mm or more. To achieve this benefit, the magnetron sputtering device must be configured to generate a plasma discharge at remote radii of the larger target to cause sputtering to occur at the remote radii.
Although the use of larger targets in magnetron sputtering devices may contribute to improved film thickness uniformity, there remain difficulties associated with the use of larger targets that have yet to be addressed. It is generally known that the sputtering process generates a substantial amount of energy in the form of heat in the sputtering target. The heat generated during sputtering needs to be dissipated; otherwise, the heat may damage the target and other components of the magnetron sputtering device.
In one previously known approach for cooling a sputtering target, a water-tight cooling chamber is formed on the side of the target opposite the target sputtering surface. The non-sputtering surface of the target forms one wall of the cooling chamber. The cooling chamber is filled with coolant (e.g., water), thus flooding the non-sputtering surface of the target with coolant. The coolant dissipates the heat generated in the target during sputtering. With this approach, the rotating magnet is immersed in the coolant.
Although a cooling chamber is effective for cooling smaller diameter targets, some difficulties arise when this approach is used for larger targets. For example, the target is subjected to a large pressure difference between the evacuated sputtering chamber on the sputtering surface side of the target and the coolant-filled cooling chamber on the non-sputtering surface side. This large pressure difference may cause the target to bow toward the substrate. A larger diameter target will bow more than a smaller diameter target, unless the larger diameter target is made thicker to withstand the greater pressure difference. However, increasing the thickness of the target increases the distance between the rotating magnet and the sputtering surface of the target, thereby undesirably reducing the intensity of the magnetic field at the sputtering surface.
Another approach to cooling a sputtering target includes forming internal cooling channels in a multi-layer target assembly as disclosed, for example, in U.S. patent application Ser. No. 08/684,440 of Anderson entitled SPUTTERING DEVICE AND LIQUID COOLED TARGET ASSEMBLY THEREFOR, assigned to Varian Associates Inc., which now abandoned is incorporated herein by reference. The cooling channels in the multi-layer target assembly may include a plurality of parallel cooling channels which connect a coolant input port to a coolant output port of the multi-layer target assembly. The parallel channels distribute coolant received through the input port throughout the target assembly to uniformly cool the target assembly. After flowing through the parallel passages, the coolant leaves the target assembly through the output port.
Highly turbulent coolant flow (e.g., Reynolds number greater than about 4000) through the cooling channels of an internally cooled target assembly is desirable to efficiently cool the target assembly. It is also desirable to keep the exit temperature of the coolant below about 60.degree. C. for safety reasons. It has been difficult, however, to obtain highly turbulent coolant flow using low coolant volume while keeping the exit temperature of the coolant below about 60.degree. C. It has also been difficult to achieve highly turbulent coolant flow in coolant channels remotely located from the coolant input port due to the pressure drop caused by coolant flowing into the coolant channels located near the coolant input port.
In the course of manufacturing a multi-layer target assembly, extreme heat and pressure may be used during the process of bonding the layers together. A difficulty that may be encountered when parallel cooling channels are used in an internally cooled target assembly is that the layer within which the grooves are formed may have a tendency to easily bend parallel to the cooling channels when extreme heat and pressure are applied. This may create defects in the bonds between the layers of the multi-layer target assembly. Use of a target assembly that incorporates such bonding defects can potentially lead to coolant leaks in the target assembly and loss of vacuum in the sputtering chamber.
In view of the foregoing, it would be desirable to provide an internally cooled target assembly for a magnetron sputtering device in which efficient target cooling is achieved through the use of highly turbulent coolant flow through cooling channels in the target assembly.
It would also be desirable to provide an internally cooled target assembly for a magnetron sputtering device in which efficient target cooling is achieved using highly turbulent coolant flow while minimizing the volume of coolant needed to cool the target.
It would further be desirable to provide an internally cooled target assembly for a magnetron sputtering device in which efficient target cooling is achieved using highly turbulent coolant flow while minimizing the volume of coolant needed to cool the target and keeping the exit temperature of the coolant below about 60.degree. C.
It would still further be desirable to provide an internally cooled target assembly for a magnetron sputtering device in which uniform target cooling is achieved using highly turbulent coolant flow through each of a plurality of parallel cooling channels in the target assembly, including cooling channels remotely located from a coolant input port of the target assembly.
It would even further be desirable to provide an internally cooled target assembly for a magnetron sputtering device that is resistant to bonding defects which may be induced by the presence of cooling channels in the target assembly.